Feedback control system for ultrasound probe

ABSTRACT

A control system for a probe, including a transmission member, comprises a power source for supplying a constant power to a transmission member and a transducer for coupling the constant power to the transmission member and for providing a mechanical output to the transmission member at a frequency. A frequency measuring device is also provided for constantly measuring the frequency of the mechanical output of the transducer. A current monitoring device for measuring current forwarded to the transducer which monitors the current while the frequency of said mechanical output is varied until it is determined at what frequency the current is at a maximum is also provided. A method for implementing this apparatus is also provided.

This application claims priority of international application No.PCT/US98/10282, which has an international filing date of May 19, 1998,now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to medical devices and moreparticularly to a method and device for delivering ultrasound energy toa treatment location within a human or other mammal.

The use of ultrasound devices for lysing or removing materialobstructing blood vessels in humans has been proposed in the art. Thesedevices use ultrasound energy, either alone or with other aspects of atreatment procedure in an attempt to remove material blocking theseblood vessels. One such device, an elongated ultrasound transmittingprobe, has been used to lyse material obstructing blood vessels ofhumans or other mammals. The device consists of a cavitation generatingtip at the end of an elongated transmission wire. A transducer is usedto convert an electrical signal into longitudinal mechanical vibrationin the transmission wire. This leads to the generation of a standingwave in the device and longitudinal displacement of the tip to transmitmechanical energy to the obstruction.

It is desirable for such an ultrasound probe to generate a wave with themaximum amplitude with a minimum of applied power. This maximumamplitude will generate the greatest lysing force and energy directed atany material being acted upon in the blood vessel. This will occur whenthe frequency of the ultrasound applied to the transmission wire of theprobe by the transducer approaches the effective resonance frequency ofthe transmission wire of the probe. However, this effective resonancefrequency will vary as the probe is moved within the blood vessel andamong different blood vessels. Thus, the transmission wire of the probemay oscillate at less than its maximum amplitude at a given appliedpower. As a result, the probe will generate less than the maximum amountof ultrasonic energy within the blood vessel. The conditions which mayaffect the probe normally include bends in the transmission wire andcompressions against the wire after the probe is fed through the variousblood vessels in the body to the obstruction and moved within the bloodvessel during treatment.

Additionally, conventional ultrasound probes do not measure the actualfrequency or amplitude of oscillation at the probe tip. For example,space concerns generally preclude the use of features to transmitinformation regarding the action of the probe tip to a user. Userstherefore will generally have no way to know what is actually happeningat the probe tip.

One effort at maintaining suitable mechanical power transmitted by thetip is described in U.S. Pat. No. 5,477,509, the contents of which areincorporated herein by reference. This reference describes attempting tocontrol the amplitude of the standing wave in the probe tip bymonitoring the current input to the transducer, and varying the powerinput to the transducer so as to maintain the current input to thetransducer at a constant level. Thus, when movement of the probe withinthe blood vessel decreases the current input to the transducer as aresult of a change in the load of the transmission wire on thetransducer, the power input to the transducer is increased in an effortto provide a constant power output at the tip of the probe. However,this reference fails to address the cause of the drop in suppliedcurrent. Rather the apparatus simply compensates for this decrease byinputting additional power. Thus, more power is required to be input tothe transducer for the same output power which results in a decrease inthe efficiency of the apparatus.

This prior art reference also describes monitoring the level of currentinput to the transducer to determine if there is a break in thetransmission wire. If a break occurs in the transmission wire, the loadof the transmission wire on the transducer will greatly decrease. Thisresults in an extreme decrease in the required power input to achievethe supposed required power output at the tip of the probe. This changesignals a problem, and the apparatus is shut down. However, such asystem will not detect a problem in the transmission wire, such as afracture, which might increase the load on the transducer. A fracturemight increase the friction between the transmission wire and any otherportion of the probe, for example, or any object the probe tip mightcome into contact with. While this fracture might be dangerous to theuser, the required power input would not decrease below a predeterminedlevel, and therefore would not be recognized as an event which wouldturn off the probe.

The optimal operating frequency of an ultrasonic device varies with thetolerances of the components of the device and the field of operation.In prior art ultrasonic devices, the optimal operating frequency isdetermined by scanning across the entire operating range of the deviceand locating the frequency which maximizes a particular operatingparameter of the device, e.g. current. A significant drawback associatedwith the prior art approach of scanning across an entire operatingfrequency range is that a false optimum frequency may be selected whichwould result in sub-optimum performance for the device.

Accordingly, it would be beneficial to provide an ultrasoundtransmission device which can generate a maximum tip oscillationamplitude under a number of adverse conditions, and provide the feedbacknecessary to maintain maximum amplitude without increasing the powerconsumption of the apparatus, and which can monitor the system to notifythe user of any fracture in the probe wire or other problem affectingthe system.

SUMMARY OF THE INVENTION

Generally speaking, in accordance with the invention, an ultrasoundtransmission apparatus in the form of a transmission member connectableto a transducer at its proximal end and having a tip at its distal endis provided. The apparatus includes an improved control system which cancontrol the amplitude of oscillation at the tip of the probe. Thiscontrol system comprises an electric power source which suppliesconstant power at a selected frequency to the transducer which convertsthe electrical energy to mechanical oscillation and generates a standingwave in the transmission member. The control system also includes afrequency measuring and adjusting instrument for continuously measuringthe frequency of the mechanical oscillations output from the transducer.This frequency measuring instrument is also capable of varying thefrequency of the oscillations of the transmission member and tip by finetuning the frequency of the oscillations generated by the transducer.Finally, current and voltage monitoring instruments are also includedfor measuring current and voltage to determine power input to thetransducer.

The control system maintains constant power (voltage times current) tothe transducer and monitors the current and voltage input to thetransducer. The oscillation frequency is varied over a predeterminedrange in order to maintain a frequency at which current input to thetransducer, and thus power, is at a maximum. The resistance along thetransmission member during oscillation is proportional to the load onthe transducer and therefore electrical resistance at the transducer isproportional to the load on the transducer. Because power is maintainedat a constant level, the load on the transducer will be at a minimum atmaximum current. The amplitude of the oscillations of the transmissionwire will also be at a maximum. Thus, as the frequency of the transduceris constantly adjusted to generate the greatest input current and thusmaintain power at its maximum, the apparatus will always optimize theamplitude of the oscillation of the tip thereof at a given power.

This maximum will occur when the transducer vibrates at the effectiveresonance frequency of the transmission member. As the probe is movedwithin blood vessels in various parts of the body, the resonancefrequency of the probe is slightly altered. By fine tuning the frequencyof the oscillation frequency of the transducer, it is possible tooscillate the transmission member at a frequency approaching this newresonance frequency. Therefore, by measuring the input current andvoltage to the transducer coupled to the transmission member while finetuning the oscillation frequency, it is possible to continuously operatethe probe at close to the resonance frequency and thus at its maximumpower. This will generate the maximum oscillation amplitude at the tipof the transmission member, and insure that the probe is being operatedunder the predetermined conditions.

Additionally, the invention includes a method for operating anultrasound transmission device, including the steps of supplyingconstant electrical power to a transducer of the device and convertingthis electrical energy to mechanical energy in the form of anoscillating tip thereof. The frequency of oscillation of the transduceris varied over a predetermined range while the current and voltagesupplied to the transducer is monitored and the power supplied to thetransducer is maintained at a constant maximum level. Then, the value ofthe frequency which results in the maximum current, and thus power beingsupplied to the transducer is determined. It is at this frequency, whichapproaches the resonance frequency of the transmission member, that theresistance to oscillation, and thus impedance of the transducer is at aminimum, and therefore the amplitude of oscillation is at its maximum.By constantly adjusting the frequency of the transducer, and constantlymonitoring for any variation in the current input and voltage to thetransducer, it is possible to maintain oscillations at the tip of thetransmission member at the appropriate amplitude, to insure appropriateultrasound application to the obstruction.

In an additional embodiment of the invention, an apparatus formonitoring the amplitude, and therefore the ultrasonic energy output byan ultrasound probe, is provided. The apparatus comprises an integrator,which receives a standard voltage input and a feedback signal indicativeof the power at the tip of the probe. This voltage signal is then fedinto a differential amplifier. This differential amplifier receivesinput from the integrator, and a feedback error signal, and generates adifferential signal which has a compensated value to maintain anaccurate frequency signal. This differential signal is then fed to a VCOphase comparator, which compares the frequency of the output signal tothe frequency of a reference signal. This reference signal is formed ofa first component which defines a predetermined, center frequency ofoscillation, and a second component which is a correction based upon thecurrent state of the system, and whether it is necessary to increase ordecrease the output frequency. This frequency is then divided by two toyield the adjusted output frequency, because the frequency hadpreviously been maintained at double the required frequency to maintaina higher degree of resolution during measurement and calculation.

This adjusted output frequency signal, which is set to the requiredfrequency, is passed through any number of power amplifiers so that theoutput signal is always maintained at a constant predetermined powerlevel regardless of the frequency or other factors. This power output isthen fed into an additional amplifier which outputs the power to atransducer, which in turn converts this electric power to a mechanicaldisplacement. At the same time, the voltage and current input to thetransducer is monitored, and the impedance is determined. These measuredvalues of voltage and current, and the determined value of impedance arefed to a multiplier/filter, which processes the signal to determine thetrue power output at the transducer, which is also a function of theamplitude of the oscillating tip of the probe. This power determinationis then fed back into the integrator where it is processed, and thefeedback control loop is completed.

Thus through the use of such an apparatus, it is possible to determinewhether the selected oscillation amplitude, and therefore, the selectedultrasonic power is being generated at the tip of an ultrasound probe.It is possible to maximize this power output by fine tuning thefrequency of the oscillations within a predetermined range, andmonitoring the transducer input current and voltage. The transduceroutput frequency which generates the greatest current, which takes placeat a frequency approaching the resonance frequency of the transmissionmember in the blood vessel, will also generate the greatest amplitude ofoscillation and therefore power output at the probe tip, withoutadjusting the input power to the transducer. Therefore, the output powerfrom a probe can be safely controlled to within a selected range withoutexpending excess power, and without sacrificing the efficiency of theapparatus.

Accordingly, it is an object of the invention to provide an improvedcontrol system for an ultrasound transmission probe.

Another object of the invention is to provide an improved control systemand method for an ultrasound probe in which the power efficiency of theprobe can be maximized.

Yet another object of the invention is to provide an ultrasound probewhich provide a constant output power.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification and thedrawings.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, and theapparatus embodying features of construction, combinations of elementsand arrangement of parts which are adapted to effect such steps, all asexemplified in the following detailed disclosure, and the scope of theinvention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference is had to thefollowing description taken in connection with the accompanyingdrawings, in which:

FIG. 1 is a side elevational view of an ultrasound probe, transducer andcontrol unit constructed in accordance with an embodiment of theinvention;

FIG. 2 is a graph depicting three of theoretical amplitude curves as afunction of transducer output frequency, for the same probe at differentlocations in a blood vessel;

FIG. 3 is a functional block diagram illustrating the procedure utilizedin operating and controlling an ultrasonic probe in accordance with anembodiment of the invention;

FIG. 4 is a block diagram depicting the functioning of a control systemconstructed in accordance with an embodiment of the invention;

FIGS. 5(a), 5(a)-1 to 5(a)-3, 5(b), 5(b)-1 to 5(b)-3, 5 c, 5 c(1) to 5c(2), 5 d, 5(d)-1 to 5(d)-3, 5(e) and 5(e)-1 to 5(e)-3 are wiringdiagrams depicting the structure of a control system constructed inaccordance with an embodiment of the invention; and

FIG. 6 is a functional block diagram illustrating the procedure utilizedin operating and controlling an ultrasonic probe in accordance with analternative embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has been determined that an effective way of lysing thrombus,occlusions and the like, is to use an ultrasound probe to deliverultrasound energy to a selected area within a patient's vasculature.However, in order to reach relatively inaccessible areas of thevasculature, it is necessary to provide a narrow and flexible devicewhich is adequately long and sufficiently guideable.

An improved ultrasound probe constructed in accordance with anembodiment of the invention for accomplishing the foregoing isillustrated generally as probe 100 in FIG. 1 hereof and in a copendingapplication entitled ULTRASOUND TRANSMISSION APPARATUS AND METHOD OFUSING SAME under application Ser. No. 08/858,247, filed May 19, 1997,the contents of which are incorporated herein by reference. Probe 100 isformed with a tapered member 112, formed with a proximal end 129 ofdiameter A_(i) coupled to a transducer 114, which acts as a source ofultrasound energy. When coupled to transducer 114, proximal end 129 ispreferably located at a displacement maximum relative to the standingultrasound wave supported by the overall device. From proximal end 129,tapered member 112 tapers, in section A thereof, to a reduced diameterdistal end 113, of diameter A_(f) at a transition zone B. Proximal end129 must be large enough to receive sufficient energy to treat thethrombus, occlusions and the like. However, in order to provide optimalflexibility, it is desirable to reduce the diameter of distal portionsof probe 100 as much as possible, without significant loss of energy,strength or guidability. Furthermore, the reduction in diameter must beaccomplished in such a manner as to amplify, i.e., increase theamplitude of, the ultrasound vibrations.

Following tapered section A of distal diameter A_(f) (or one or moretapered sections A), is a constant diameter section C, of diameterC_(i), where C_(i)<A_(f). In the event additional reductions in diameterare desired, a second transition zone D can be provided, for couplingsection C to a section E of one or more lengths of transmission media,each of diameter E_(i), where E_(i)<C_(i). Each of these sections Athrough E comprise a transmission member for delivery of ultrasoundenergy to selected locations within the vasculature and otherwise. Itshould also be understood that transmission members having constructionsdifferent than that of device 100, including unitary transmissionmembers and otherwise can also be employed with the control system andmethod of the invention.

Section C may be composed of a different material than Section A. Forexample, Section A may be composed of aluminum formed as a wire or rodor other appropriate structure which has superior ultrasoundtransmission properties, is easily machined and is inexpensive, andSection C may be composed of titanium, titanium alloys or othermaterials that have adequate ultrasound transmission properties andgreater strength for the same diameter.

In accordance with preferred embodiments of the invention, Section A, ifit includes a taper, preferably has a tapered length which is equal toan integral multiple of half wavelengths of the intended frequency ofoperation. At the terminus of Section A, there may be a transition zoneB, which is a step transition, wherein Section C has diameterC_(i)<A_(f). To effect maximum displacement amplification,step-transition zone B should be placed at or near a displacement node(i.e., a displacement minimum). Thus, if Section A includes a taperedsection which is an integral multiple of half wavelengths, it should befollowed by a straight section of length equal to an odd multiple (i.e.1, 3, 5 . . . ) of quarter-wavelengths. In this way, Section A begins,at the proximal end 129 at a displacement maximum, and ends at itsdistal end 113 at a displacement minimum (displacement node). If SectionA is straight (i.e. constant diameter), then it should begin at adisplacement maximum and terminate at a displacement node.

Device 100 also includes a mass or cavitation tip 115 at the distal tipthereof Cavitation tip 115 is designed and shaped to distributeultrasound energy and/or perform work in accordance with the applicationof interest. As a standing wave is generated in device 100, tip 115 willoscillate longitudinally and transmit ultrasound energy. The larger theamplitude of oscillation at a particular frequency, the greater thepower output.

Ultrasound device 100 (as well as other probes formed with a structuresimilar thereto) is understood to operate in the resonance frequencymode; i.e., it supports a standing wave (preferably a longitudinal wave)when energized by ultrasound stimulation at proximal end 129.Consequently, it is preferred that cavitation tip 115 is located at adisplacement maximum (anti-node). Transition zone D may be located at adisplacement node or anti-node. For example, transition zone D mayinvolve a joint that couples several parallel lengths of transmissionmedia, of diameter E_(i), to section C. In that case, it may bedetermined that the mechanical strength of transition zone D isinsufficient to support maximum stress. For such a case, transition zoneD may be located at or near a displacement maximum (stress minimum).

It is understood that the techniques for controlling the probe andassembling the sections thereof are equally applicable to systems thatpromote or focus ultrasound energy to enhance the absorption of drugs,reduce apoptosis in cells, and/or treat tissue, tumors, obstructions,and the like, within and without the body, systems to be utilized in forlaproscopic surgery, and ultrasonic scalpels, for example.

During the use of an ultrasound probe in accordance with the invention,the ultrasound energy can be generated by the linear oscillation of atip of a transmission member, such as a wire, at a particular frequencyand amplitude. When this amplitude is at a maximum, for a predeterminedoscillation frequency, the ultrasonic output power generated by thisoscillation is also at a maximum. Therefore, an objective of efficient,safe operation is that an ultrasound probe is always operated close tothis maximum amplitude. In a preferred embodiment, this oscillationmaximum at the tip of the probe is within a range of 20 to 150 microns,more preferably between 20 and 100 microns, and most preferablyapproximately 40 microns.

It has been determined that when an ultrasound probe is fed throughblood vessels or other objects, the required bends and turns of theprobe and other reasons associated with the geometry required by theprobe when passing through the blood vessel of a human or other body,the resistance and load of the transmission member on the transducerincreases. When operated, the transmission member is oscillated in astanding wave. A standing wave includes standing nodes and anti-nodes.The oscillation amplitude is the greatest at the anti-nodes, while thereis little or no displacement at the nodes. As the probe is moved withina blood vessel, pressure from different directions on the probe andother environmental changes, affect the resonance frequency of thetransmission member. Thus, when constructing a probe in accordance withthe invention, it is advantageous to construct an environment similar tothe environment which will be encountered during use in order to selectthe desired range of driving frequencies.

By adjusting the frequency of the ultrasound output from the transducer,within a predetermined range, it is possible to approach the effectiveresonance frequency of oscillation of the transmission member so that itcoincides with the resonance frequency of the member in the currentposition and shape. Thus, by being able to adjust this oscillationfrequency, when the output amplitude, or output power is decreasedbecause of movement of the probe within the body, rather than increasingthe input power to compensate for this reduction in output power, thefrequency can be varied slightly until the maximum power output isachieved. This will occur when the actual frequency of oscillation isequal to the effective resonance frequency of the probe. Thus ratherthan simply applying extra power to compensate for power loss in thesystem, which could overload the system, as has been done in the priorart, the invention attempts to address the source of the decreased poweroutput, (in this case, oscillation of the probe wire at other than theresonance frequency) thereby improving power output without increasingpower input, and also reducing the risk of damage to the blood vessel inwhich the probe is situated, the probe itself or otherwise.

As is noted above, however, it is difficult to directly measure theactual oscillation amplitude at the tip of a probe. Therefore, a systemin accordance with the invention can utilize an alternative measurement,which is representative of the oscillation amplitude, and thereforeultrasonic power output, at the tip of the probe. By utilizing threewell known formulae in which V is voltage, I is current, and Z isimpedance:

(1) Power=VI

(2) V=IZ

it follows that

(3) Power=I²Z.

Therefore, if power is kept constant, any increase in the resistance,measured as an increased impedance will result in a decrease(non-linear) in the current supply. Any events which affect theresonance frequency of the transmission member and increase thedifference between the resonance frequency thereof and the actualoscillation frequency of the transducer will effectively increase theresistance to mechanical oscillation of the transmission member. Thisresults in increased electrical impedance at the transducer.Consequently, because R (resistance) and Z (impedance) are inverselyproportional to I (current), any event which will adversely affect theamplitude of the mechanical oscillations of the transmission member canbe detected by an accompanying decrease in the current flow to thetransducer. Thus, as the difference between the resonance frequency ofthe transmission member and the actual oscillation frequency of thetransducer (as a result of a change in the resonance frequency), thecurrent flow to the probe will decrease.

Such a situation is depicted in FIG. 2, which shows amplitude ofoscillation on the Y-axis as a function of frequency of the transduceron the X-axis. Curve 200 is formed with a maximum at approximately themiddle thereof, and minim at each end thereof. Thus, for curve 200,frequency 250 results in a maximum amplitude. Frequency 250 is theresonance frequency for the probe at one location. Curve 200 representsthe frequency/amplitude response curve for an idealized positioning of aprobe within a blood vessel in a body. In a preferred embodiment thisresults in an optimum frequency of approximately 42 kHz. As the probe ismoved within the blood vessel, the frequency/amplitude response curveshifts. Therefore, curve 200 can shift to the values of curve 210 if theaction performed on the probe reduces the resonance frequency tofrequency 251, or curve 200 can shift to the values of curve 220, if theaction performed on the probe increases the resonance frequency of thetransmission member to frequency 252. It is to be understood that thelocations of curves 200, 210 and 220 are only used as examples, and thatfrequency/amplitude response curves exist for each resonance frequencyof oscillation of the transmission member.

Thus, after movement of the probe, and an accompanying shift in thefrequency/amplitude response curve, the actual frequency of theoscillation of the transmission member will no longer be at theresonance frequency. Therefore, the amplitude of oscillation will nolonger be at a maximum. As is shown in FIG. 2, if the frequency responsecurve is shifted from curve 200 to curve 210, whereas oscillationfrequency 250 corresponds to the maximum current and amplitude of curve200, it is now at lower arm 240 of curve 210, at a location less thanthe maximum current and amplitude. Therefore, if the oscillationfrequency from the transducer were decreased, it would be possible toapproach the resonance frequency of the transmission member, and therebymove to a position 233 corresponding to the maximum current andamplitude of the new curve.

In order to adjust the frequency, the steps as set forth in FIG. 3 maybe followed. First, in step 1, the oscillation frequency output from thetransducer is determined. Next, in step 2, the current level input tothe transducer for this particular frequency of oscillation is measured(I₁). These two characteristics form the base line information of thecurrent system. Then in step 3, the frequency of oscillation of thetransducer is increased a predetermined amount (to the right in FIG. 2)and the current at this second frequency (I₂) is measured in step 4. Ina preferred embodiment, this predetermined frequency change is 75 Hz.Then, similarly in step 5, the frequency of oscillation of thetransducer is decreased a predetermined amount, (to the left in FIG. 2)and the current at this third frequency (I₃) is measured in step 6. In apreferred embodiment, this predetermined frequency change is 75 Hz. Instep 7, the current measured at the second frequency (I₂) is compared tothe original current (I₁). If the current measured at the secondfrequency is less than at the original frequency (I₂<I₁), then theprocess moves to step 8 where the current at the third measuredfrequency (I₃) is compared to the current at the original frequency(I₁). If this current at the third frequency is also less than theoriginal frequency (I₃<I₁), then since both increasing and decreasingthe frequency correspond to a decrease in the current, the current isalready at the maximum. Therefore in step 9, since the amplitude willalso be at a maximum, the frequency is not changed. Then, the procedurereturns to step 1 for measurement of the frequency again at the nextsampling time.

If, however, at step 8, the current at the third frequency had beengreater than at the original frequency (I₃>I₁), then in step 12, the newfrequency is set to the third frequency, and control shifts back to step1.

If at step 7, it is determined that the current measured at the secondfrequency is greater than at the first frequency (I₂<I₁), then controlpasses to step 10. In step 10, if the current at the third frequency isnot greater than the current at the second frequency (I₃<I₂), then atstep 11, the new frequency is set to the second frequency. If thecurrent at the third frequency is greater than the current at the secondfrequency (I₃>I₂), then in step 12 the new frequency is set to the thirdfrequency. After these steps, control is returned to step 1.

It is possible to perform this sampling routine at any selected timeinterval. The more frequently the values are sampled, the more accuratecontrol of the probe will be. In a preferred embodiment of theinvention, sampling is performed within a range of approximately morethan every 50 milliseconds, preferably more than every 25 milliseconds,and most preferably approximately every 13 milliseconds.

In the example as depicted in FIG. 2, if the resonance frequency were todecrease to frequency 251, the frequency/amplitude curve would shiftlocations from curve 200 to curve 210. The frequency and amplitude wouldmeet at point 230, below the maximum amplitude 233 for thefrequency/amplitude curve 210, and also below the maximum current forthe frequency/amplitude curve, and not at the new resonance frequency251 of the transmission member. Following through the steps in FIG. 3,the current at a frequency higher than point 230 would be measured, andthe current at a frequency at a point lower than point 230 would bemeasured. It would be determined that the current at the frequency belowpoint 230 would be greater, and the frequency would be lowered. Thisprocess would continue until the frequency reached point 232, 233. Atpoint 233, neither the second nor the third frequency would produce acurrent greater than that at point 233. Thus, the frequency would notchange since the current at that frequency would be at a maximum. If thefrequency were at point 234 on curve 210, the same procedure would befollowed, only during each iteration, it would be determined that thefrequency should be increased to increase the current, and therefore theamplitude.

If the frequency increases or decreases are chosen to be large enough,it is possible that the frequency changes will pass from over thefrequency corresponding to the maximum current and amplitude from oneside of curve 210 to the other, without stopping at the maximum. In apreferred embodiment, the frequency changes are approximately 150 Hz,more preferably 100 Hz, and most preferably 75 Hz, although other valuescan be used, based upon the geometry and other characteristics of thesystem. In this case, the algorithm will simply change the frequency inthe other direction to obtain a substantially maximum current andamplitude. In a preferred embodiment, when two consecutive measurementsindicate that the frequency should be changed in two differentdirections, it can be determined that the frequency corresponding to themaximum current and amplitude has been passed by. Thus, it is possibleto take an average of these last two measured frequencies to determinethe approximate optimal frequency. Alternatively, it would be possibleto reduce the size of the current increase or decrease at each step tofocus in on the maximum current. Thus, by using larger current changesat first, and then using small changes when the current is close to themaximum, the maximum is reached more quickly, and more accurately.

Under the process described above in which the full operating frequencyof the probe is sampled, the time required to determine the optimalprobe operating frequency and whether a power mismatch exists can beapproximately 25 seconds. It is desirable to reduce this time as much aspossible so that performance and system safety is improved and to ensurethat a broken probe is not damaged further. Accordingly, in analternative embodiment, the full operating frequency range of the probeis divided into a minimum of three frequency subranges with eachfrequency subrange having a center frequency. The center frequencies foreach subrange are selected based on an analysis of the tolerances of theprobe, transducer and control unit and the field of operation of theprobe, all of which affect the location of the center frequencies andhow they are maintained.

It has been found that in a coronary probe, the preferred firstfrequency subrange has a first center frequency of approximately 41.6kilohertz, the preferred second frequency subrange has a second centerfrequency of approximately 41.9 kilohertz, and preferred the thirdfrequency subrange has a third center frequency of approximately 41.3kilohertz. It is been found that by sampling for the optimal probeoperating frequency successively within these three frequency subranges,the optimal probe operating frequency and the presence of a powermismatch can be determined more quickly, often within 15 to 20 seconds.

In order to determine the optimal probe operating frequency in thealternative embodiment, the steps as set forth in FIG. 6 may befollowed. First, in Step 1, the frequency output of frequency generator435 is set to the first center frequency of the first frequencysubrange, the probe is energized and a differential amplifier/VCO phasecomparator 425 causes the frequency output of frequency generator 435 tosample frequencies in the range of ±150 Hz around the first centerfrequency. Next, in Step 2, the power input to the transducer ismeasured. Next, in Step 3, the maximum power input measured in Step 2 iscompared to the minimum level necessary to operate the probe safely,which in a preferred embodiment of the invention is approximately 80% ofa predetermined power level (18 watts in one embodiment). If the maximummeasured power input is greater than 80% of the predetermined value,then the frequency at which this power input level is achieved is usedto operate the probe. At this point, the process repeats Step 2 tocontinuously monitor that the power input to the transducer remains atthe minimum operable power level. If however, in Step 3, a sufficientpower input level is not initially detected, the systems waitsapproximately 5 seconds to determine if the power level of the probewill reach the minimum operable power level as a result of impedancechanges due to placement of the probe within the vessel. If the minimaloperable power level is not detected after 5 seconds, the processproceeds to Step 4 in which the frequency output of frequency generator435 is set to the second center frequency and the second frequencysubrange is tested. As in steps 2 and 3, in Steps 5 and 6 the powerinput to the transducer is measured and the maximum power input measuredis compared to the minimum level required to run the probe. If asuitable frequency at which to operate the probe is not found in thesecond frequency subrange, the third frequency subrange is selected andtested in Step 7-9. If no suitable frequency is located at which theprobe can operate safely in the third frequency subrange, in Step 10 apower mismatch flag is set and the probe is de-energized.

In an alternative embodiment of the invention, this iterative processmay be changed slightly. Specifically, rather than increasing anddecreasing the frequency from the original frequency, measuring thecurrent at each frequency, and then changing the current in theappropriate direction, it is possible to measure and calculate the slopeor phase angle of the frequency/amplitude curve at the current frequencylocation. Based upon this measurement, it would be determined in whichdirection the slope increases, and the frequency of the transmissionmember oscillation could be adjusted accordingly. When the slope of thecurve is determined to be flat or zero, the frequency would be producinga maximum current, and therefore amplitude, and would not need to beadjusted.

In an additional embodiment of the invention, it is possible toconfigure the control system to also monitor for any irregular events inthe system, including the fracture or breakage of the transmission wire,or any other event which might effect the effectiveness or safety of thesystem. Specifically, if the transmission wire were to break, the loadof the transmission wire on the transducer will decrease. This will inturn result in an extreme change in resonance frequency as well as anincrease in the current supplied to the transducer while maintaining aconstant power input to the transducer, and in turn, the controlapparatus will attempt to compensate by greatly shifting the oscillationfrequency of the transducer. However, when the transducer oscillationfrequency or current is no longer within a predetermined range ν−νΔ andν+Δν, the control apparatus determines that there is a problem with thesystem, and can shut the probe down. In a preferred embodiment, thisrange includes values from 20 to 100 kHz, more preferable from 30 to 45kHz and most preferably in the range of 42 kHz±500 Hz. Thus, it ispossible to monitor or correct the system for an unexpected, drasticchange in the required frequency of oscillation or current in order toshut down the probe is there is a problem.

Additionally, a problem in the transmission wire, such as a fracture,could increase the load on the transducer. This will in turn result in adecrease in the current supplied to the transducer while maintaining aconstant power input to the transducer, and in turn, the controlapparatus will attempt to compensate by shifting the oscillationfrequency of the transducer. However, when the transducer oscillationfrequency is no longer within the predetermined range, (preferably 42kHz±500 Hz) the control apparatus will determine that there is a problemwith the system, and can shut the probe down. Thus, it is also possibleto monitor the system for an unexpected, drastic change in the requiredfrequency of oscillation as a result of an increase in resistance, whichwould also result in a decrease in current supplied to the transducer inorder to shut down the probe when there is a problem.

FIG. 4 is a block diagram depicting the functioning of a control systemconstructed in accordance with one embodiment of the invention. A blockdiagram of an apparatus for monitoring the amplitude, and therefore theultrasonic energy output by an ultrasound probe, is indicated generallyas control system 400. Control system 400 comprises a processor controlapparatus 410 for controlling the interaction of each of the operationsperformed by system 400. A start element 415 receives a signal fromcontroller 410 and begins the process. A Gating/Integrator 420 receivesa standard voltage input, ramping at low frequency, and therebygenerates a voltage from 0V to a predetermined limit. In a preferredembodiment, this predetermined limit is 10V. A feedback error signal 476indicative of the power at the tip of the probe is also received atintegrator 420, as will be discussed below. Power is supplied in apreferred embodiment by a 165 volt DC source.

Signal 421 from integrator 420 is fed into Differential Amplifier of aDifferential/VCO Phase Comparator 425. This differential amplifierreceives input from integrator 420 and feedback error signal 476 andgenerates a differential signal which has a compensated value tomaintain an accurate frequency signal. This differential signal is thenfed to a VCO Phase Comparator, also depicted within block 425, whichcompares the frequency of the output signal to the frequency of areference signal. This reference signal is generated by a firstcomponent signal from center frequency generator 435, which defines apredetermined, center frequency of oscillation, and a second componentsignal from a frequency adjuster 430, which is a correction based uponthe current state of the system, and whether it is necessary to increaseor decrease the output frequency. Frequency generator 435 and frequencyadjustor 430 comprise a variable frequency generator, in a preferredembodiment. This calculated frequency signal 426 is then forwarded toPower A/D 440, which is monitored by controller 410 to maintain thesystem at the optimum frequency, and frequency divider 445, where thisfrequency is divided by two to yield the adjusted output frequency. Thefrequency had previously been maintained at double the requiredfrequency, to maintain a higher degree of resolution during measurementand calculation. This divided frequency signal 446 is also forwarded toa Frequency Counter 450, which allows controller 410 to monitor thefrequency signal which will be output from the system.

The adjusted output frequency signal 411, which is set to the requiredfrequency, is first passed through an Amplitude Control/Filter 455,which level shifts and references the signal to the predetermined setpower levels. The signal is AC coupled by gating signal 456 and filteredto provide a bipolar signal at the system operation frequency. Thisbipolar signal inputs into a Drive Amplifier 460. Drive Amplifier 460amplifies the bipolar signal from Amplitude Control/Filter 455. In apreferred embodiment, the filtered bipolar signal is amplified with again of 2. Then, this output is forwarded to an amplifier, a PowerAmplifier Out and Current and Voltage Sensors PAO/CVS 465. Power Amp Out465 further amplifies the filtered bipolar signal to be transmitted to aTransducer Out 470, which will be converted to mechanical energy in theform of a mechanical displacement. This transducer may be apiezoelectric transducer, in a preferred embodiment. This power outputsignal is always maintained at a constant predefined power duringoperation, regardless of the frequency or other factors. In onepreferred embodiment, the predetermined power is 18 watts.

At the same time, the voltage and current input to the transducer aremonitored at PAO/CVS 465, and the impedance is determined based upon thestate of the probe. The measured values of current and voltage are fedto a Multiplier/Filter 475, which processes the signal indicative of themeasured values to determine the true power output at the transducer,which is also a function of the amplitude of the oscillating tip of theprobe. The current and voltage sensors may both be implemented astransformers. This power determination signal 476 is then fed back intothe gating integrator 420 where it is processed, and the feedbackcontrol loop is completed. This power determination is then utilized todetermine whether the oscillation frequency of the probe tip should bealtered. The system utilizes the method as set forth in FIG. 3 for thisdetermination.

Reference is next made to FIGS. 5(a)-5(d), which depict specificstructure of a preferred embodiment of the invention which may beemployed to implement the invention as shown in FIG. 4. It is to beunderstood that any additional components not specifically mentioned arealso included in the preferred embodiment, as are depicted in thefigures. Any reference to any specific components is similarly intendedto be for example only, and is in no way intended to limit thestructures which may be used herein.

Controller 410 is a computer controller, and may utilize any computerwith sufficient controller software instructions to control thefunctioning of the feedback control apparatus. Gating/Integrator 420performs a gating and integration function, and is depicted in FIG.5(c). Gating/Integrator 420 includes an NPN transistor package 501, andNPN/PNP transistor package 502, a QUAD comparator 503, an operationalamplifier 504 acting as a buffer, an operational amplifier 505 acting asan integrator, and an analog switch 506. These components are wired asshown in FIG. 5(c). In a further preferred embodiment, a particular chipwhich may be employed as NPN transistor package 501 is sold by Motorolaunder the designation MMPQ3904. A particular chip which may be employedas NPN/PNP transistor package 502 is sold by Motorola under thedesignation MMPQ6700. A particular chip which may be employed as QUADcomparator 503 is sold by Motorola under the designation LM239. Aparticular chip which may be employed as operational amplifiers 504 and505 is sold by Linear Technology under the designation LT1212. Analogswitch 506 is sold by Motorola under the designation HC4066.

Differential Amplifier/VCO Phase Comparator 425 performs the calculationof the actual frequency, compares this to the desired frequency andproduces a differential signal, which allows for the adjustment of theoutput frequency, and is depicted in FIG. 5(a). DifferentialAmplifier/VCO Phase Comparator 425 includes a phase locked loop 507, a10K Digital POT 508 calculating the frequency offset from the desiredfrequency, a 50K Digital POT 509 controlling the frequency range aboutthe desired frequency, and an operational amplifier 510 acting as adifferential amplifier. These components are wired as shown. Aparticular chip which may be employed as Phase Locked Loop 507 is soldby Harris under the designation CD4046B. A particular chip which may beemployed as 10K Digital POT 508 and 50K Digital POT 509 are sold byDallas Semiconductor under the designation DS1267-10 and DS1267-50respectively. A particular chip which may be employed as operationalamplifier 510 is sold by Motorola under the designation LT1212. Alsoshown in FIG. 5(a) is Center Frequency Generator 435, which includes ahigh frequency waveform generator 511, which generates a waveform at apredetermined desired frequency. A particular chip which may be employedas high frequency waveform generator 511 is manufactured by Maxim underthe designation MAX038.

Frequency Adjuster 430 is shown in FIG. 5(d) and includes a frequencycontroller 512 which controls and adjusts the center frequency, wired asshown. A particular chip which may be employed as frequency controller512 is sold by Burr-Brown under the designation DAC7801. FIG. 5(d) alsodepicts Power Analog to Digital converter 440, which includes a digitalto analog converter 513 and which interfaces with controller 410 formonitoring power, and Frequency Counter 450, which includes atimer/counter 514 and which interfaces with controller 410 to monitoroutput frequency, wired as shown. A particular chip which may beemployed as digital to analog converter 513 is sold by Burr-Brown underthe designation ADC7802. A particular chip which may be employed astimer/counter 514 is sold by Intel under the designation 82C54.

As is further connected as shown in FIG. 5(a), divide by 2 means 445includes a frequency divider 515, Amplitude Control Filter 455 includesan Operational Amplifier 516 acting as a control filter, and DriveAmplifier 460 includes an operational amplifier 517 acting as a driveamplifier. A particular chip which may be employed as frequency divider515 is sold by National Semiconductor under the designation CD4013. Aparticular chip which may be employed as operational amplifier 516 oroperational amplifier 517 is sold by Linear Technology under thedesignation LT1212.

Power Amp Out/Current and Voltage sensors 465 include a drivetransformer 518, a voltage feedback transformer 519 and a currentfeedback transformer 520, as shown and connected in FIG. 5(e). FIG. 5(e)also depicts Transducer 470, which includes a power transformer 521,connected as shown.

Finally, Multiplier/Filter 475 is depicted and connected as shown inFIG. 5(b), and includes a 10K Digital POT 522, which sets the currentand voltage gain, an Analog Multiplier 523, which calculates the power,an Operational Amplifier 524, which acts as a filter and an operationalamplifier 525, which act as a current and voltage buffer. A particularchip which may be employed as 10K Digital POT 522 is sold by DallasSemiconductor under the designation DS1267-10. A particular chip whichmay be employed as Analog Multiplier 523 is sold by Burr-Brown under thedesignation MPY634. Particular chips which may be employed asOperational Amplifiers 524 and 525 are sold by Linear Technology underthe designation LT1212.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in carrying out the above method andin the constructions set forth without departing from the spirit andscope of the invention, it is intended that all matter contained in theabove description and shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

What is claimed:
 1. A method for controlling a probe including atransmission member and a transducer for generating mechanicaloscillations and generating a standing wave on the transmission member,comprising: supplying a constant power to a transducer coupled to atransmission member and generating a standing wave on the transmissionmember; varying the frequency of oscillation of the transmission membercoupled with said transducer by a first amount above a selectedfrequency; varying the frequency of oscillation of the transmissionmember by a second amount below the selected frequency; measuring thecurrents supplied to said transducer while the frequency is varied bythe first and second amounts; and adjusting the frequency of oscillationaccording to the currents measured while the frequency is varied by thefirst and second amounts.
 2. A control system for a probe coupled to atransducer constructed to oscillate at a selected frequency and impartoscillation to a transmission member, comprising: a power source forsupplying a constant predetermined electrical power; a transducercoupled to the power source for converting the electrical power tooscillation at a selected frequency and coupled to a transmission membercapable of supporting a standing longitudinal wave; a frequencyadjustment device for measuring the frequency of the mechanical outputof the transducer; and varying the frequency a selected amount above andbelow said selected frequency; a current monitoring device for measuringcurrent supplied to the transducer which monitors said current while thefrequency of said mechanical output is varied; and a processor operableto control the frequency adjustment device and the current monitoringdevice wherein under the control of the processor, wherein the frequencyadjustment device varies the frequency of said mechanical output by afirst amount above said selected frequency and by a second amount belowsaid selected frequency and wherein the selected frequency is adjustedaccording to the currents monitored by the current monitoring devicewhile the frequency adjustment device varies the frequency by the firstamount and by the second amount.
 3. The control system of claim 2,wherein said transmission member includes a wire or a rod.
 4. Theapparatus of claim 2, wherein said power generator is a 165 volt DCsource.
 5. The apparatus of claim 2, wherein said transducer is apiezoelectric transducer.
 6. The apparatus of claim 2, wherein saidselected frequency is approximately 42 kHz.
 7. The apparatus of claim 6,wherein said frequency is varied at most ±500 Hz above and below saidselected frequency.
 8. The apparatus of claim 2, wherein said frequencyadjustment device is a controller of a variable frequency generator. 9.The apparatus of claim 2, wherein said current monitoring device is atransformer.
 10. The apparatus of claim 2, wherein said voltagemeasuring device is a transformer.
 11. The apparatus of claim 2, whereinsaid frequency selector is a variable frequency generator.
 12. A controlsystem for a probe coupled to a transducer constructed to oscillate at aselected frequency and impart the oscillation to a transmission member,the control system comprising: a power source that supplies anelectrical power; a transducer coupled to the power source to convertthe electrical power to oscillations at a selected frequency and coupledto a transmission member capable of supporting a standing longitudinalwave; a frequency adjuster operable to vary the transducer outputfrequency; a current monitor that measures current supplied to thetransducer while the frequency of the transducer output is varied by thefrequency adjuster; and a processor coupled to and operable to controlboth the frequency adjuster and the current monitor, wherein under thecontrol of the processor, wherein the frequency adjuster varies thefrequency of the transducer output by a first amount in a firstdirection and by a second amount in a second direction opposite to thefirst direction while the current monitor measures the currents suppliedto the transducer and wherein the selected frequency of the transduceroutput is adjusted according to the currents measured by the currentmonitor.
 13. The control system according to claim 12, wherein under thecontrol of the processor, the frequency variation by the frequencyadjuster in the first and second directions and adjustment of theselected frequency are repeated continuously to optimize the oscillationamplitude.
 14. The control system according to claim 12, wherein underthe control of the processor, the frequency variation by the frequencyadjuster in the first and second directions and adjustment of theselected frequency are repeated continuously to optimize the oscillationamplitude and the first and second amounts are each at most 500 Hz. 15.The control system according to claim 12, wherein the transmissionmember includes a wire or rod operable to oscillate by the transducerand coupled to the transducer for insertion into a human body.